In the rest of the description, the expressions “part made of metal material” and “part made of composite material” are called “metal part” and “composite part” respectively.
It is known how to use a process of bore expansion and interference-fit mounting on a fastener to increase the fatigue life in an assembly between two metal parts. These expansion and interference processes cause compressive stresses on the surface of the bore and locally in the material of which the part is composed. These stresses have the effect of slowing down the initiation and propagation of fatigue cracks in the immediate vicinity of the bore. The result is an increase in fatigue life. There are several expansion processes, and they can be used separately or in combination.
FIG. 1A shows a known so-called interference mounting process for assembling two metal parts that consists of making a bore hole in the two parts 1 having a diameter Øbore less than a fastener diameter Øfastener which is the diameter of the fastener rod 9. Inserting the fastener 9 in the bore generates compressive stresses on the periphery of the bore holes because the fastener diameter Øfastener is greater than the bore diameter Øbore. The stresses created in this way make it possible to increase the fatigue life in these work areas, [which are] critical because of the initiation of cracks in areas with loads.
FIG. 1.B is a schematic view of another example of the known process of generating stresses locally on the surface of the bore in an assembly of metal parts. An expansion process is used with an expansion tool 6. The process includes the following steps:                (1) the first bore is made with a conventional boring tool 5 in the two metal parts; the diameter of the bore is chosen so that the diameter of the bore is adapted to the diameter of the expansion tool 6, i.e., the bore diameter Øbore must be slightly less than the diameter of the expansion tool.        (2) next the expansion tool 6, a so-called burnisher, is passed through the bore made in step (1); this tool has an olive-shaped part with a diameter greater than that of the bore; its passage through the bore strain-hardens the bore and generates residual compressive stresses on the surface of the bore and locally in the material of the two parts.        (3) a final bore is made to adapt the diameter of the holes to the diameter of the fastener rod, and then the fastener is put on to hold the assembly of the two parts.        
Parts made of composite material have exceptional properties in terms of resistance to mechanical fatigue and strong rigidity, while giving structures a very low weight. These parts are used especially in the aeronautics industry, including in structures with very heavy loads. However, assembling these composite parts poses specific problems compared to metal parts.
Indeed, the composite parts 2, as shown in FIG. 1.C, are comprised of structures obtained by stratification of fibers impregnated with resin, for example carbon fibers impregnated with an epoxy resin. Such a composite part has structural properties that are advantageous in the fiber strata plane, but sensitive to delamination phenomena in a direction perpendicular to the planes, i.e., in the direction of the bore used to put on the fastener.
The compressive forces exerted by the means of attachment can cause the phenomenon of delamination in the bore. In general, to prevent this delamination phenomenon, it is necessary to minimize the stresses that appear in the fastener area, that is, at the interface between the wall of the bore hole and the fastener. To do so, contrary to the case of a metal assembly, a bore with a diameter slightly greater than the diameter of the fastener 9 is generally made so as to leave enough play between the wall of the bore hole and the surface of the fastener 9 to prevent interference.
In aeronautic structures, the coexistence of metal and composite parts leads to frequent assembly of metal parts with composite parts. There can be junctions between two panels of different structures or local reinforcement, for example ribs, or metal stiffeners on a composite panel.
In such an assembly, either an assembly with play between the wall of the holes and a fastener is chosen, and the mount is then unfavorable to the metal part in terms of fatigue life, or interference-fit assembly is chosen and such mounting risks damaging the composite part.
One solution would be to make a hole in the metal part and in the composite part, which are two distinct parts, for the fastener, then generate a field of residual stresses in the metal part in the absence of the composite part, and then place the composite part against the metal part for assembly. This solution is not satisfactory, indeed in this case, it is necessary to predetermine precisely the positions of the holes so that they are aligned during assembly for the passage of the fastener. This alignment cannot be done industrially.